Situation: I ready an action to __BLANK__ her as she starts casting.
Its unclear to me what if any affect the following Combat Maneuvers have against a caster to interrupt their spells with a readied action (I am assuming the caster is casting defensively):
Bull Rush - on success they are moved back 5 feet+ - the spell still goes off?
Dirty Trick - on success the caster spends a move action to remove the affect - does this somehow interrupt the spell?
Disarm - on success the caster is disarmed - can you disarm their spell components/divine focus? The spell still goes off if the item is not required to cast the spell.
Drag - on success you drag the caster 5 feet+ back - the spell still goes off?
Grapple - on success damage done increases Concentration check.
Overrun - on success the caster is overrun - the spell still goes off?
Reposition - on success the caster if moved - the spell still goes off
Steal - on success you steal an item from the caster - can you steal their component pouch before they cast - so unless their spell required the item you stole the spell still goes off?
Sunder - on success you sunder the caster's item they have in hand - but unless that item was required to cast the spell the spell still goes off?
Trip - on success you trip the caster - does the spell still go off?
Feint - on success you feint the caster - does nothing to increase Concentration check because you do no damage
Bullrush or Trip or Reposition or Drag would qualify as Violent Movement for Concentration. Greater Trip also lets you take an AoO, which would trigger the Damage while Casting Concentration Check, besides the Violent Motion one.
You could Sunder or Disarm the spell component they are using for the spell, which would make the spell fail IMHO (Spell Casting includes drawing the component, not drawing any and all components). You could also use the Steal or Sunder Manuever on their Spell Component Pouch BEFORE they´ve taken their ingredient from it, for similar effect unless they have a second Spell Component Pouch. Greater Sunder can also apply damage to the Caster themself, also triggering Concentration, though this doesn´t seem necessary if they don´t have the component in hand any more.
Initiating a Grapple doesn´t NORMALLY do any damage, but the Concentration check is now based on your CMB, making it tough to Cast (and they can´t Move - but since you´re NOT depending on readied/Interrupt damage to ruin their spell, you might as well Ready for the start of their turn or just Grapple them on their own turn, so they can´t 5´ step away).
Dirty Trick may impose Spell Failure Chance depending on the effect achieved - look up their Conditions in the Glossary or Magic Casting section. Feint does nothing, and isn´t technically a combat maneuver anyways. Next time, try NOT putting an empty line in between item lists like that, it doesn´t help readability and makes your post take up way too much space.
Bullrush or Trip or Reposition or Drag would qualify as Violent Movement for Concentration.
What's the penalty for that?
Also, if the target is out of range (say, a healing spell), it could be a problem.
Only if you are out of actions. Otherwise, finish casting the spell, move in, and cure your patient.
We houseruled that the caster just makes a Concentration check equal to the DC the attacker made (d20 + 5 + CMB + any bonuses the attacker has for that Maneuver), when there is no damage done.
For the Violent Motion modifier, is it a base of 15 or 20 for the Concentration check? For example would Bull Rushing a caster 5-feet be a DC 20+Spell Level or would you need to push the 10+ feet?
It´s totally a GM call, the rules don´t say.
I would say that allowing Bullrushes of 10´+ to count as Extremely Violent (20+ Spell Level) seems reasonable, but the rules don´t discern here... Beyond that a Bullrush would seem to inherently qualify as ´violent motion´ per normal English grammar.